FIG. 1 is a longitudinal front view showing a typical miniature motor to which this invention is applied. In FIG. 1, reference numeral 1 refers to a case made of a metallic material, such as mild steel, formed into a bottomed hollow cylindrical shape and having permanent magnets 2 formed into an arc-segment shape fixedly fitted to the inner circumferential surface thereof. In the case 1 housed is a rotor 5 comprising an armature 3 facing the permanent magnets 2 and a commutator 4. Numeral 6 refers to a case cap made of a synthetic resin or other appropriate insulating material and formed in such a manner as to fit to an open end of the case 1. Numeral 7 refers to brush arms adapted so as to make sliding contact with the commutator 4. The brush arms 7 are housed in the case cap 6, together with input terminals 8 electrically connected to the brush arms 7. Numerals 9 and 10 refer to bearings provided on the bottom of the case 1 and the central part of the case cap 6, respectively, to rotatably support a shaft 11 constituting the rotor 5.
With the aforementioned construction, as current is fed from the input terminals 8 to the armature 3 via the brush arms 7 and the commutator 4 constituting the rotor 5, rotating force is imparted to the armature 3 existing in a magnetic field formed by the permanent magnets 2 fixedly fitted to the inner circumferential surface of the case 1, causing the rotor 5 to rotate, driving external equipment (not shown) via the shaft 11 on the output side.
In miniature motors of the conventional type having the aforementioned construction, brushes (not shown) provided on the tips of the brush arms 7 and the commutator 4 are subject to abnormal wear in a short period of time due to arc discharge, joule heat, or contact between the brushes and the commutator 4 at a sliding-contact area between the brushes and the commutator 4. If this abnormal wear occurs, electrical connection between the brushes and the commutator 4 could not be maintained, deteriorating expected stability in motor performance and reducing the service life of the miniature motor.
When a miniature motor is used in an atmosphere containing organic gases, a black-colored insulating polymer is produced on the sliding-contact area between the brushes and the commutator 4 due to arc discharge and joule heat, as mentioned above. This insulating polymer, when produced, would make the state of contact between the brushes and the commutator 4 unstable, increasing contact resistance. This would result in reduced motor life.
To solve these problems, a method of encapsulating polyvalent alcohol, etc. in a motor case to produce an atmosphere of polyvalent alcohol, etc. in the motor case has been proposed (Japanese Published Unexamined Patent Application No. Sho-60(1985)-162449, for example). Even this method, however, has not sufficient effects of extending service life in a miniature motor having such a construction as shown in FIG. 1.